Thach Hen lake is located in Tra Linh district, only one hour from Cao Bang town, from Cao Bang town, and enjoying the cool air of Thang Trang lake. The lake is located between a rocky area 800-1000m above sea level and over 500m above the terrain, but never dry out during the dry season while the lakes are in the rainy season.

The lake's 36 reservoirs are separated by several tens or hundreds of meters of lagoons, each with its own boundary but interconnected by underground caves, 36 of which are naturally lakes. In a vast valley adjacent to Quoc Toan, Tra Linh and Ngu Lao communes, Hoa An are named after the local dialects such as Thang Vuong, Na Ma, Thang Loong, Thang Loi. now on. In that population, Thang Hen is the largest lake with a length of 2000m, width 50m surrounded by old forests, interwoven with cat's ear accent in Thang Khanh language meaning "Ong Tail", viewed from above. High visitors will associate the shape of the lake as the bee's tail, in the lake there are more than 100 kinds of fish, different small shrimps forming a rich seafood.

Legend of Thang Lake is an interesting discovery of tourists, the legend has it that in the past in Cao Bang there was a guy named Sung smart guy who passed the test and was awarded the King, 7 days honored Before he was appointed as the mandarin of the country, he married beautiful girl Ooc, adored the beautiful wife that chang Sung forgot the day of business, Saturday night to remember his wife divorce and My parents ran for business, in the middle of the night the wilderness ran 36 footsteps and fell head to the mountain and died, 36 footsteps today are 36 large and small lake with different names of dialects in the district Trails, it is said that where he lay down is Thang Lake today.